Ans: There are structures located inside the nucleus of animal and plant cells which look like spiral threads, they are called Chromosomes. These are made of a single molecule of DNA and protein. Parents pass them to their children while DNA contains specific instructions to make each type of living creature (plants, animals or anything) unique.
The Greek words chroma (colour) and soma (body) are the origin of the term Chromosome. Scientists gave this name because they are strongly stained by colourful dyes used in research.

Chromosomes have such a distinctive structure that helps it stay wrangled around spool-like histone proteins. DNA molecules would be too long to fit inside cells without such packaging. E.g., if the DNA molecules inside a cell were unwounded it’d be 6 feet in length!
Cells must keep dividing to produce new cells to replace old cells for organisms to grow and function properly. DNA have to remain intact and evenly distributed among cells while cell division takes place. Chromosomes play a key role in successfully copying and distributing DNA accurately in the vast majority of cell divisions.
Irregularity in number or structure of chromosomes cause severe problems. E.g., some kinds of Cancers are taken place because of joined pieces of broken chromosomes.
It is also crucial that reproductive cells, such as eggs and sperm, contain the right number of chromosomes and that those chromosomes have the correct structure. If not, the child will be ill-developed. E.g., in Down syndrome, people have three copies of 21st Chromosome instead of two.

There are 23 pairs (46) of Chromosomes in human cell.
In fact, each species of plants and animals has a set number of chromosomes. Like a fruit fly has four pairs of chromosomes, while a rice plant has 12 and a dog, 39.
How are chromosomes inherited?
In the more developed organisms and humans, the male and female parent donate one copy of each chromosome in a pair. This is why children have features from both their father and mother.

Yes, they are different. A pair of sex chromosomes or allosomes are different in them. There are two X chromosomes in females while males have one X and one Y.